A known inkjet printing apparatus comprises two conveyors, one supplying a sheet of paper or other material, to a transfer area and one receiving the sheet after being transferred from the supplying conveyor. The supplying conveyor, such as a print belt conveyor having a transport belt for advancing the sheet along an inkjet print station, is arranged for advancing the sheet in a transport direction to a transfer area where the sheet is transferred to the receiving conveyor. Said print station may be arranged for applying an inkjet image onto a process side of the sheet using an ink, such as by applying dots of an aqueous ink.
The supplying conveyor may have a suction mechanism arranged for providing a suction force to attract a contact side of the sheet towards the transport belt, the contact side being opposite to the process side. As such, a holding force is provided for holding the sheet during driving in the transport direction towards the receiving conveyor along with the movement of the transport belt of the supplying conveyor.
The receiving conveyor comprises a transport belt having a support surface for supporting the sheet at a contact side of the sheet. The receiving conveyor may further comprise a suction mechanism arranged for providing a suction force to attract the contact side of the sheet towards the support surface of the transport belt of the receiving conveyor.
When the print station forms an inkjet image on the process side of the sheet by applying dots of an aqueous ink the printed sheet becomes wet due to the aqueous ink dots. The moisture is absorbed into the sheet and enlarges the fibers of the sheet at the process side of the sheet, depending on the sheet properties. As a result, the sheet may curl at the side edges and/or the corners of the sheet and most of the time said curl is downward curl deformation towards the contact side of the sheet. Said curled sheet may provide several problems during transfer of the sheet to the receiving conveyor.
First the downward curled sheet may provide problems during a separation step of the sheet from the transport belt of the supplying conveyor. In a known separation step, the printed sheet is separated from the transport belt by applying a separation air flow to a leading edge of the sheet proximate to a deflection element, such as a roller, which deflection element deflects the transport belt of the supplying conveyor. Said separation air flow is directed along the transport belt adjacent to the deflection element for lifting the leading edge of the sheet from the transport belt at the position of the deflection element.
However a downward curled portion of the sheet, which curl develops after the holding force of the suction pressure is removed, makes it more difficult to separate the leading edge of the sheet from the transport belt. It has been found, that the leading edge of the sheet may be pushed backwards at the deflection element by the separation air flow in a direction opposite to the transport direction, thereby further bending the downward curled portion of the sheet towards the transport belt. As a result, the sheet is not reliably separated from the transport belt, and in the case it is separated, it is not reliably transferred to the receiving conveyor.
Furthermore when the sheet is transferred to the receiving conveyor, the downward curled sheet may land first at curled side edges and/or corners on the transport belt of the receiving conveyor. At the paint the suction mechanism of the receiving conveyor attracts the contact side of the sheet wrinkles may grow in the sheet as the sheet is not allowed to sufficiently flatten on the support surface of the transport belt of the receiving conveyor. These wrinkles may even become fixed in the sheet during a drying step of the printed sheet in a drying unit arranged downstream of the sheet transfer assembly.
In general it is known to flatten an edge portion of a curled sheet by passing an air flow from the center of the sheet over curled edges of the sheet in a direction substantially parallel to a support surface in order to flatten the curled edges of the sheet. However, in all of these steps during transfer of the sheet from the supplying conveyor to the receiving conveyor, curl deformation level of sheets has been found not to be static but time dependent. Furthermore curl deformation may vary considerably over type of sheets and over process conditions, such as environmental conditions, storing conditions of the sheets, process temperature, process speed and ink amount.
When applying such an air flow over a sheet at a lower air flow level, the air flow level may be insufficient to suppress downward curl deformation for a sheet, especially when having a strong curl deformation. Furthermore when applying said air flow over a sheet at a high air flow level, the air flow may disturb the transfer of the sheet, such as not being able to separate a flattened sheet from the transport belt of the supplying conveyor and/or disturbing the alignment of the sheet in the transport direction due to an increased friction between the sheet and the support surface.